4. Concepts and Comparisons

Any new invention, per definition, has its advantages and disadvantages. For a new product to be commercially accepted, it must improve on what has come before. However, it is not enough just to improve on one aspect at the detriment of the other. For example, a Diesel engine is more efficient than a petrol engine, but it has problems with emissions. A Wankel engine produces a lot of power for it’s size, but it is not efficient. We hope to show in this chapter how the D.A.R.T. engine has been conceived as a complete and no compromise package.

4.1 Why Rotary?

The conventional internal combustion engine has been with us for over 100 years. During this time it has altered very little. Mechanically, it still converts oscillatory motion into rotary motion, which is intrinsically inefficient. Attempts to make this design more efficient usually entail adding extra components and complexity, e.g. turbo charging, supercharging etc. As a general concept, if what you want is rotary motion then why not start with rotary motion. Our philosophy is to keep things simple, no crankshafts, connection rods, pistons, cams and multiple valves. Several notable designs have been put forward, but for one reason or the other, be it technical, environmental or efficiency grounds, they have not reached general popularity. The D.A.R.T. design not only overcomes all the previous problems encountered with other rotary engines but addresses the fundamental principles that make an engine more powerful, efficient, economical, and non-polluting.

4.2 Efficiency

All engines, no matter what type, are governed by the laws of thermodynamics. The efficiency of any engine depends on the difference between the highest and lowest temperatures reached during one cycle. The greater the difference, the greater the efficiency. An engine would be more efficient if the fuel/air mixture burned at a higher temperature and the exhaust gases exited the engine at a lower temperature. In most engines the compression volume equals the expansion volume, therefore a significant amount of energy escapes with the exhaust gases. If however, the expanding gases were allowed to fill a larger volume, then the pressure and temperature of the gases would be less at the end of this phase. Diagram 4 shows schematically the volumes for a conventional piston engine, the D.A.R.T. engine and a Wankel engine respectively.

Diagram 4: Size comparisons

The expansion volume of the D.A.R.T. engine is larger than the compression volume, allowing complete expansion of the gases. Thermodynamically this allows the maximum amount of chemical energy from the fuel to be converted into mechanical work. Both the conventional piston engine and the Wankel engine are constrained by their fixed symmetrical geometries in this respect and require extra components in order to retrieve some of the lost energy.

4.3 Asymmetric Geometry

The asymmetric geometry of the D.A.R.T. engine not only controls the volumetric properties but also the kinematics. In normal symmetrical engines, the volume change and mechanical advantage are directly related (a no win situation). The D.A.R.T. engine is not constrained by a symmetrical geometry, compression and expansion volumes are independent of each other and the mechanical advantage values are much higher. A comparison of Unit Torque (the mechanical advantage) v Chamber Volume is shown in chart 1.

Chart 1: Volume Vs Torque

The asymmetric geometry of the D.A.R.T engine causes a phase shift between the volumetric curve and the Unit Torque curve. A comparison with a symmetrical piston engine is depicted in Chart 2.

Chart 2: Mechanical advantage

For the D.A.R.T. engine the unit torque curve is much larger in the expansion phase. An asymmetry in the volumetric curve also causes the chamber pressure curve to be shifted from the compression region into the expansion region. This is shown in Chart 3.

Chart 3: Chamber pressure

Chamber pressures are generally lower for the D.A.R.T. engine, except for the first 15 degrees of the expansion phase. This allows the fuel/air mixture to combust at a higher temperature for a given compression ratio. Due to the greater pressure drop in the expansion phase, the exhaust gases exit the engine cooler, thus fulfilling the first requirement for thermodynamic efficiency. The actual amount of torque that the engine can produce is the product of the chamber pressure and the mechanical advantage. The resulting torque curve is shown in Chart 4.

Chart 4: Torque

The asymmetric geometry of the D.A.R.T. engine produces significantly more torque than the symmetrical arrangement of a conventional piston engine. Note the duration of compression and expansion events are shorter for the D.A.R.T. engine.

4.4 Power and Size

The D.A.R.T. engine produces more power than a conventional engine in three different ways. Firstly, the total amount of work that is done in one revolution is more for the rotary engine. Secondly, the rotary engine has twice the number of power phases. (One power phase per revolution as opposed to one power phase per two revolutions for the conventional engine.) Thirdly, by nature of its rotary construction the maximum rpm can be higher, i.e. more power phases per minute. The Wankel engine also has one power phase per revolution, but due to the geometry of the engine (a low compression ratio together with a long and narrow combustion chamber) its efficiency is lower than a conventional engine.

A two-bank D.A.R.T. engine will have the same number of power phases as a four-cylinder four-stroke conventional engine operating at the same rpm. Unlike the conventional engine the D.A.R.T. engine devotes most of its internal volume to the induction process. This results in an extremely compact power unit. For the same capacity, the D.A.R.T. engine would be only half the size/weight of a conventional engine, yet still produce 27% more power.
Diagram 5 shows the comparative sizes of both engines.

Diagram 5: Internal volume

The extra rpm range of the D.A.R.T. engine will also yield a pro rata increase in power. Only the internal volumes of both engines are shown, no valve gear, ancillaries etc. We think the chart speaks for itself, a quite remarkable reduction in size for an engine of the same induction capacity, is it not?

4.5 Combustion Chamber

Unlike most rotary engine designs this new engine possesses a true combustion chamber. This is fundamental to the design and has important implications. Ideally a combustion chamber should do just that, provide the optimum conditions for the combustion of the air/fuel mixture. In a conventional engine the combustion chamber also houses the valves and its shape is governed to a large extent by their inclusion. The combustion chamber on the D.A.R.T. engine is free from valves and can be shaped to provide turbulent compression of the air and optimum mixing of the injected fuel. Long flame paths and excessive heat conducting surfaces associated with the Wankel engine are avoided. Significant increases in torque are obtained with the new engine whilst the rotor driving faces are within the confines of the combustion chamber.

4.6 Fuel and Economy

Why Diesel? The D.A.R.T. engine will function well with any fuel, and the concepts already mentioned would apply equally well to whichever fuel is used. An engine has to convert the chemical energy stored in the fuel, into rotational work. Using Diesel as a fuel has particular advantages in this respect. A conventional Diesel engine is 50% efficient, this compares with 30% efficiency for a petrol engine. It is expected that the D.A.R.T. engine can attain 63% efficiency by using Diesel as a fuel. This is within the efficiency range of current fuel cell technology. Diesel fuel is relatively inexpensive, it is in plentiful supply for the near future and does not present the problems of transportation and storage associated with fuels like hydrogen. Up to now the major drawback with Diesel fuel have been the emissions, which will be discussed in the next chapter.

Apart from the D.A.R.T. engines intrinsic efficiency, which requires less fuel per kilometre, the engine itself is only half the weight/size of a conventional engine, thus packaging the engine and all its components (ancillaries, fuel tank, etc.) makes the vehicle much smaller and lighter. This all reduces the fuel requirements substantially. The extended rpm range and tractability of the engine saves weight/size in the transmission components. With twice as many power phases per revolution, the D.A.R.T. engine can operate at half the idle speed of a conventional engine, offering enhanced economy in stop-go situations.

4.7 Air Scavenge and Emissions

All the cycles of a conventional engine are completed within one revolution of the D.A.R.T. engine. Most of these cycles happen in parallel with each other. There are two compression phases in the new engine. The first compression phase controls the later scavenge of the exhaust gases. This scavenge phase has several functions. Firstly, it helps to oxidise the exhaust gases more fully. Secondly, it reduces the temperature of the exhaust gases. Thirdly, the scavenge air can be re-circulated within the engine to act again in the scavenge phase. Finally, it avoids an extra phase for the mechanical expiration of the exhaust gases.

Air contains not only oxygen but also mainly nitrogen. During the combustion process of converting hydrocarbons and oxygen into carbon dioxide and water, the nitrogen also tends to react with the oxygen forming nitrogen oxides (NOx). Traditionally exhaust gas recirculation (EGR) has been used as a means of reducing peak temperature during combustion. Since the exhaust gas does not participate in the combustion process it absorbs some of the energy and hence lowers the peak temperature and reduces NOx formation. The pulsed air scavenging of the D.A.R.T. engine provides copious amounts of excess air. A high excess air ratio allows a greater use of EGR to further reduce NOx production. The scavenge phase provides a high-velocity air stream and turbulent mixing of the combustion by-products. The increased oxygen concentration further enhances particulate matter (PM) oxidation and helps burn up the PM as they form. Because NOx is formed early in the combustion cycle, adding air late in the cycle does not increase NOx. It is expected that with the amount of excess air almost doubled that both NOx and PM are reduced simultaneously. Since Diesel engines operate at an overall lean fuel-air ratio, they tend to emit low levels of hydrocarbons (HC) and carbon monoxide (CO). Excess air will however show a reduction in HC and CO emissions. Because the D.A.R.T. engine is based on high excess air capacity it is not sensitive to fuel sulphur content.

The regulated emissions for new engines are hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and particulate matter (PM). Unlike petrol engines, Diesel engines do not have much trouble meeting HC and CO standards. Traditionally the problem emissions have been NOx and PM. With the unique air scavenge characteristics of the D.A.R.T. engine, emission levels from both these sources can be significantly reduced.

Diesel fuel requires a higher compression ratio than petrol for the combustion process, which is not obtainable in most rotary designs. Rotary engines that can use Diesel fuel do exist, but they have not fulfilled the basic principles as previously mentioned. The D.A.R.T. design however, combines all the advantages of Diesel fuel with the simplicity, compactness and smoothness of a rotary engine. All the basic principles have been tackled head-on and without compromise.

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